Conventional seat structures for vehicles, especially seat structures for automobiles generally include seat back frames as the frames of seat backs supporting occupants. Such a seat back frame is formed as a single casting piece obtained by casting an aluminum alloy. Moreover, an attachment portion of a retractor (a supporting portion) is supported near a side edge of the seat back frame on the sidewall of the vehicle body. A reinforcement portion configured to support load acting on the seat back in the longitudinal direction of the vehicle body is provided for a part extending from the attachment portion of the retractor to the lower end of the seat back frame in the vertical direction (see PTL 1).
As for the frame body including the seat back frame and a seat cushion frame, an iron plate is punched out by pressing and is then subjected to drawing to form a deep beed and the like. The frame body thus has enough strength to support an occupant sitting on the seat (see PTL 2).